1. Field of the Invention
The present invention relates to a film bulk acoustic device, using an acoustic wave, such as resonators and filters.
2. Description of the Prior Art
FIGS. 1 and 2 show this type of the conventional film bulk acoustic wave device as described in, for example, the literature xe2x80x9cBulk-Acoustic-Wave Devices using the Second-Order Thickness-Extensional Mode in Thin ZnOxe2x80x94SiO2 Composite Films on Sixe2x80x9d, Proceedings of The Acoustical Society of Japan, pp. 691-692, September-October 1985 (hereinafter, referred to as reference 1), and the literature xe2x80x9cFundamental Bulk Acoustic Resonators in GHz Rangexe2x80x9d, Proceedings of The Institutes of Electronics, Information and Communication Engineers, vol.81, No.5, pp.468-472, 1998 (hereinafter, referred to as reference 2). FIG. 1 is a top view, and FIG. 2 is a cross sectional view taken along line Ixe2x80x94I in FIG. 1. In the figures, reference numeral 1 denotes a silicon (Si) substrate; 2 denotes a silicon oxide (SiO2) formed on the silicon substrate 1; 3 denotes a bottom electrode formed on the silicon oxide 2; 4 denotes a piezoelectric film composed of zinc oxide (ZnO) formed on the bottom electrode 3; 5 denotes a top electrode, divided into input-side electrode 5a and output-side electrode 5b, formed on the piezoelectric film 4; and 6 denotes a via hole.
When a voltage is applied to the top electrode 5 and the bottom electrode 3, an electric field is generated in the piezoelectric film 4. At this time, an acoustic distortion in the piezoelectric film 4 is caused by the electric field. When the applied voltage is a signal of frequency f, this distortion also vibrates at the same frequency f and excites an acoustic wave. In the case of a structure as shown in FIGS. 1 and 2, the excited acoustic wave propagates in the thickness direction, and the acoustic wave propagated in the thickness direction is reflected on the respective air-contact surfaces on the surface of the top electrode 5 and the lower surface of the silicon oxide 2. Therefor, there occurs an acoustic resonance when a gap between the surface of the top electrode 5 and the lower surface of the silicon oxide film 2 is an integer multiple of the half wave length of the acoustic wave.
On the other hand, there also occurs the propagation of the acoustic wave in the direction parallel to the surface in the interior of the piezoelectric film. The acoustic wave in the piezoelectric film 4 at this moment comes to be a cut-off mode at a frequency lower than a certain frequency f0, and comes to be a propagation mode at a frequency higher than the frequency f0. The frequency f0 is a cut-off frequency, and corresponds to a frequency in which the thickness 2 h of the piezoelectric film 4 coincides with a half wavelength of the acoustic wave propagating in the thickness direction in the piezoelectric film 4 when the two surfaces of the piezoelectric film 4 are free surfaces.
In the case of the electrode unit in which the top electrode 5 is present on the surface, the cut-off frequency fm0 of the electrode unit is lower than the cut-off frequency ff0 of a non-electrode unit provided with only the bottom electrode 3, due to the electrode thickness and mass load. Therefor, in the range of the frequencies ff0 to fm0, the acoustic wave is the propagation mode due to a higher frequency side than the cut-off frequency fm0 in the electrode unit, while it is the cut-off mode due to a lower frequency side than the cut-off frequency ff0 in the non-electrode unit. Hence, the acoustic wave propagating in parallel to the surface of the film 4 results in a state that an energy is trapped in the top electrode 5.
When the input-side electrode 5a and output-side electrode 5b of the top electrode 5 are set appropriately at a symmetric mode frequency providing the common potential, and at an asymmetric mode frequency providing potentials different from each other, an electric signal applied to the input-side electrode 5a is traveled to the output-side electrode 5b with low loss. Thus, the pass band of a filter is created.
The characteristics of such a filter are determined by the thickness 2 h of the piezoelectric film 4, the thickness of the top electrode 5, the thickness g of the silicon oxide 2, the shape of the top electrode 5, and a gap between the input-side electrode 5a and the output-side electrode 5b. 
The reference 1 describes one example in which a normalized thickness (g/h) of the piezoelectric film 4 is 1.54 and 2.4, which is determined by the thickness 2 h of the piezoelectric film 4 and the thickness g of the silicon oxide 2. The reference 1 also represents that the design is made by focusing the temperature characteristics and the loss of the acoustic wave. In addition, the reference 1 designates to use an acoustic wave of the second mode. The acoustic wave is a fundamental mode (first mode) when a gap between the surface of the top electrode 5 and the back of the silicon oxide 2 is a half wave length of the acoustic wave, and an N-th mode (N: integer) corresponds to an N multiple of the wave length of the fundamental mode.
FIG. 3 shows this type of the conventional film bulk acoustic device described in, for example, JP-A 6-350154 (hereinafter, referred to as reference 3). FIG. 3 is a cross sectional view. Though the basic structure is the same as that shown in FIG. 2, a piezoelectric film 7 is composed of lead titanate-zirconate (PZT); a bottom electrode 3 is composed of a titanium (Ti) film 8 and a platinum (Pt) film 9; and a top electrode 5 is composed of a titanium film 10 and a gold (Au) film 11.
The reference 3 describes one example in which a normalized thickness (d/h) of the bottom electrode 3 is 1.0, which is determined by the thickness 2 h of the piezoelectric film 7 and the thickness d of the bottom electrode 3. In addition, the reference 3 designates that favorable piezoelectric characteristics may be performed by an appropriate composition ratio of lead titanate (PbTiO3) and lead zirconate (PbZrO3).
The piezoelectric film 7 composed of lead titanate-zirconate excites a thickness extension vibration as a main vibration. In this case, the acoustic wave propagating in parallel with the surface of the piezoelectric film 7 designates the dispersion characteristics as shown in FIG. 4. In FIG. 4, the horizontal axis corresponds to a normalized thickness of the piezoelectric film 7 which multiplies the wave number k of the acoustic wave propagating in parallel with the surface of the film 7, and the thickness of the piezoelectric film 7 together, that is, a normalized piezoelectric thickness (2 kh), while the vertical axis corresponds to a frequency.
In the figure, reference numeral 12 designates the characteristics of a first mode (TE1) of the thickness extension vibration; 13 designates the characteristics of a second mode (TS2) of the thickness shear vibration; 14 designates the characteristics of a third mode (TS3) of the thickness shear vibration; and 15 designates the characteristics of a second mode (TE2) of the thickness extension vibration. A range that the normalized piezoelectric substance thickness is a real number is a range that the acoustic wave is a propagation mode, while a range that the normalized piezoelectric substance thickness is an imaginary number is a range that the acoustic wave is a cut-off mode. In addition, the frequency at the crossing point with the vertical axis, such that the normalized thickness is zero, is a cut-off frequency f0.
As is apparent from FIG. 4, the first mode (TE1) of the thickness extension vibration shows a characteristic that the frequency is made lower as the normalized piezoelectric substance thickness is larger in the vicinity of the vertical axis. In addition to lead zirconate titanate, this is also applied similarly in a piezoelectric film having the thickness extension vibration as a main vibration, constituting lead titanate (PbTiO3), lithium tantalate (LiTaO3), or a material having the Poisson""s ratio of one-third or less.
When a characteristic like the first mode (TE1) of the thickness extension vibration in FIG. 4 is exhibited, the cut-off mode corresponds to a frequency higher than the cut-off frequency fm0 in a range that the top electrode 5 exists, while the propagation mode corresponds to a frequency lower than the cut-off frequency ff0 in a range that the top electrode 5 is absent. Therefor, a favorable energy trapping cannot be performed, resulting in enlarged loss. The variations of the characteristics of the filter may be enlarged under the influence of the ambience of the top electrode 5.
As a method to prevent such a drawback, a method as described in JP-B 58/58828 (hereinafter, referred to as reference 4) has been used. That is, an addition is added to lead titanate-zirconate, so that the Poisson""s ratio of the lead titanate-zirconate mixed with the addition can be one-third or more. Instead of the characteristics like the first mode (TE1) of the thickness extension vibration in FIG. 4, it is set to show up the following characteristics: the frequency is higher as the thickness of the normalized piezoelectric substance is larger in the vicinity of the vertical axis, thereby performing the same energy trapping as a case where the piezoelectric film 4 composed of lead oxide are applied. Note that when the Poisson""s ratio exceeds one-third, the cut-off frequency of the first mode (TE1) of the thickness extension vibration is higher than that of the second mode (TS2) of the thickness shear vibration.
This type of the conventional film bulk acoustic wave device which enables to perform the energy trapping is limited by a piezoelectric film composed of a material such as zinc oxide which generates the thickness shear vibration as a main vibration. For this reason, there is a drawback such that it is difficult to flexibly correspond to a variety of characteristics necessary for a filter.
In the piezoelectric film composed of a material such as lead titanate-zirconate, having the thickness extension vibration as a main vibration, since it is difficult to perform the energy-trapping as it stands, the energy-trapping has been performed by bringing the Poisson""s ratio to one-third or more with mixing an addition to the material. But, the method described in the reference 4 can be performed only by a fabrication method which sinters piezoelectric ceramics. When the method is applied to a piezoelectric film, it is difficult to maintain favorably the quality of the piezoelectric film, resulting in deteriorating the characteristics of film bulk acoustic wave devices.
The present invention has been made to solve the aforementioned drawbacks, and an object of this invention is to obtain a film bulk acoustic wave device which enables an energy trapping with a piezoelectric film for exciting or generating a thickness extension vibration as a main vibration, thereby providing a more favorable characteristic than the prior art.
A film bulk acoustic wave device according to the present invention comprises: a substrate; a dielectric film including a silicon nitride (SiN) formed on the substrate and a silicon oxide (SiO2) on the silicon nitride; a bottom electrode formed on the dielectric film; a piezoelectric film formed on the bottom electrode; and a top electrode formed on the piezoelectric film, wherein a via hole is formed in such a manner that the thickness direction of a part of the substrate which is opposite to a region including a part where the top electrode exists is removed from the bottom surface of the substrate to a boundary surface with the silicon nitride.
Thus, the breakdown of the components upon the etching to form a via hole may be prevented through the silicon nitride, and simultaneously, the inferiority of the characteristics of the components due to the deformation caused by the inner stress upon forming the via hole may be prevented.
In the film bulk acoustic wave device according to this invention, the silicon oxide is formed separately on the silicon nitride and on the top electrode.
Thus, the errors on fabrication may be adjusted by the silicon oxide formed on the top electrode, and simultaneously, the top electrode may be protected.
In the film bulk acoustic wave device according to this invention, the piezoelectric film generates a thickness extension vibration as a main vibration, and in order to prevent the thickness of the silicon nitride from affecting the vibration characteristics, the thickness of the silicon oxide is larger than that of the silicon nitride, and 1.5 times or more the thickness of the piezoelectric film.
Thus, an energy trapping possible film bulk acoustic wave device may be performed, which has an effective electromechanical coupling coefficient k2 larger than this type of the conventional film bulk acoustic wave device and has the characteristics of wider range and reduced loss, thereby preventing unnecessary spurious occurrences caused by the end shape of the piezoelectric substance originally irrelevant to the characteristics of the film bulk acoustic wave device, a positional relationship between the end of the piezoelectric substance and the end of the via hole, and so on, and further preventing enlarged loss when the energy of an acoustic wave propagates to the outside of the top electrode.
A film bulk acoustic wave device according to this invention comprises: a dielectric film; a bottom electrode including a conductor having a density of 10000 (kg/m3) or more; a piezoelectric film which generates a thickness extension vibration as a main vibration; and a top electrode including a conductor having a density of 10000 (kg/m3) or more, wherein when the thickness of the piezoelectric film is set to 2 h, the thickness of the dielectric film is set to g, the density of the top electrode is set to xcfx811xc3x971000 (kg/m3), the thickness of the top electrode is set to d1, the density of the bottom electrode is set to xcfx812xc3x971000 (kg/m3), the thickness of the bottom electrode is set to d2, and an equivalent density R is set to R=(xcfx811d1/h)+(xcfx812d2/h), the normalized thickness (g/h) of the dielectric film determined by the thicknesses of the piezoelectric film and the dielectric film is {0.15xc3x97R+2.8} or more.
Thus, the film bulk acoustic wave device may perform an energy trapping by use of the second mode (TE2) of the thickness extension vibration, and further has a larger electromechanical coupling coefficient than this type of the conventional film bulk acoustic wave device and little occurs a thickness shear vibration other than the thickness extension vibration as a main vibration, thereby achieving the characteristics of a wider range and a lower loss than the prior art.
A film bulk acoustic wave device according to this invention comprises: a dielectric film; a bottom electrode including a conductor having a density of 10000 (kg/m3) or more; a piezoelectric film which generates a thickness extension vibration as a main vibration; and a bottom electrode including a conductor having a density of 10000 (kg/m3) or less, wherein when the thickness of the piezoelectric film is set to 2 h, the thickness of the dielectric film is set to g, the density of the top electrode is set to xcfx811xc3x971000 (kg/m3), the thickness of the top electrode is set to d1, the density of the bottom electrode is set to xcfx812xc3x971000 (kg/m3), the thickness of the bottom electrode is set to d2, and an equivalent density R is set to R=(xcfx811d1/h)+(xcfx812d2/h), the normalized thickness (g/h) of the dielectric film determined by the thicknesses of the piezoelectric film and the dielectric film is {0.023xc3x97R+3.5} or more.
Thus, the film bulk acoustic wave device may perform an energy trapping by use of the second mode (TE2) of the thickness extension vibration, and further has a larger electromechanical coupling coefficient than this type of the conventional film bulk acoustic wave device and little occurs a thickness shear vibration other than the thickness extension vibration as a main vibration, thereby achieving the characteristics of a wider range and a lower loss than the prior art.
In the film bulk acoustic wave device according to this invention, the dielectric film has a silicon nitride (SiN) formed on the substrate, and a via hole is formed in such a manner that the thickness direction of a part of the substrate which is opposite to a region including a part where the top electrode exists is removed from the bottom surface of the substrate to a boundary surface with the silicon nitride.
Thus, the breakdown of the components upon the etching to form a via hole may be prevented through the silicon nitride, and simultaneously, the inferiority of the characteristics of the components due to the deformation caused by the inner stress upon forming the via hole may, be prevented.
In the film bulk acoustic wave device according to this invention, the silicon oxide is formed separately on the substrate and on the top electrode.
Thus, the error of the thickness upon fabrication may be adjusted and simultaneously, the top electrode may be protected.
In the film bulk acoustic wave device according to this invention, the bottom electrode is mainly composed of either platinum (Pt) or iridium (Ir).
Thus, a favorable piezoelectric film may be obtained.
In the film bulk acoustic wave device according to this invention, the piezoelectric film has as a main component either one of lead titanate (PbTiO3), lead titanate-zirconate (PZT), lithium tantalate (LiTaO3), and a material having the Poisson""s ratio less than 0.33.
Thus, a flexible countermeasure may be performed in accordance with the characteristics required for the film bulk acoustic wave device.